America’s Most Challenging High Schools ranks schools through an index formula that’s a simple ratio: the number of Advanced Placement, International Baccalaureate and Advanced International Certificate of Education tests given at a school each year, divided by the number of seniors who graduated that year. A ratio of 1.000 means the school had as many tests as graduates.

Students in a lab at the Signature School, a charter school in Evansville, Ind., that ranked in the Top 10 on this year's list. (Photo by Terry W. Hughes/Courtesy of the Signature School)